The Process of Synaptic Transmission

The process of synaptic transmission, including reference to neurotransmitters, excitation and inhibition

Description – AO1:Synapse: When there are chains of neurons, there are (tiny) gaps between them i.e. they don’t actually touch. Any neuron will have 2 synapses:

1. Where the dendrites from another neuron connect to it.

2. Where its axon terminal connects with the dendrites of another neuron.

• Initially, the electrical nerve impulse travels down the neuron and prompts the release of neurotransmitters (chemicals in the brain) at the presynaptic terminal.
• These chemicals are then released into the synaptic fluid of the synapse.
• The adjacent neuron must then quickly take up the neurotransmitter from the fluid and convert this into an electrical impulse to travel down the terminal to the next pre-synaptic terminal (allowing the impulse to be transmitted on).
• This process occurs at high speed.

Labelling a Synapse

Interesting Research: Yamoto and Kitazawa (2001) examined why we cannot easily perceive when we are touched in two places simultaneously. For example, if someone touches your shoulder and toe at exactly the same time it feels as if each is being touched at a slightly different time. This is argued to be because of the inability of the nervous system to transmit that information accurately as the distance from the brain for the neurons receiving the message is different.

It should be noted however, that not all messages prompt activation in the same way. It depends on the action-potential of the post-synaptic neuron and the message type received.

Only certain neurotransmitters can unlock a message channel in certain receptors in the post-synaptic neuron (operates under a lock and key system – only certain keys will release the lock). When the right key (neurotransmitter) meets the right lock (receptor) a specific ion channel in the membrane is opened (like a door). Ions then flow through the membrane into the neuron along their specific pathways. This flooding of ions can cause a ‘potential’ in the dendrites. These potentials can be excitatory or inhibitory.

(1) Excitatory Potentials:
Make it more likely for the neuron to fire and so, if a synapse is more likely to cause the post-synaptic neuron to fire it is called an excitatory synapse.
An excitatory neurotransmitter binding with a posy-synaptic receptor causes an electrical charge in the membrane of that cell resulting in an expiatory post-synaptic potential (EPSP).

For example, noradrenaline is an example of an excitatory neurotransmitter as a result, it is the ‘on switch’ of the nervous system – this increases the likelihood that an excitatory signal is sent to the post-synaptic cell, which is then more likely to fire.

(2) Inhibitory Potentials:
Make it less likely to fire and, if the message is likely to be stopped at the post-synaptic neuron, it is called an inhibitory synapse.

Inhibitory neurotransmitters are generally responsible for calming the mind and body, including sleep and filtering out unnecessary excitatory signals.
For example, GABA AND Serotonin are examples of inhibitory neurotransmitters – they are the nervous systems ‘off switches’ in that they decrease the likelihood of neurons firing.

Summation:
The excitatory and inhibitory influences on the post-synaptic neuron are summed (added together), if the net effect on the post-synaptic is inhibitory, the neuron will be less likely to ‘fire’ and if the net effect is excitatory, the neuron will be more likely to ‘fire.’